Text 3 Application Technology

Ultrasonic metal welding is a proven technology, especially when workpieces need to be bonded on a permanent basis without consuming significant heat. Since no fusion takes place, the process is also applicable for welding materials with different melting points.

Reasons for ultrasonic metal welding

When it comes to selecting this technology, the following advantages should also be considered:

• there is no coarse grain formation and thus no embrittlement of the parts

• mechanical stability and resistance to corrosion remained constant

• an excellent electric conductivity is guaranteed

• no welding consumables are required

• special ambient conditions (i.e. vacuum) are not necessary

• negligible electric power is consumed in comparison to other welding processes

Materials suitable for ultrasonic welding

The materials which a most suitable for ultrasonic metal welding are nonferrous metals and their alloys of any combination. Therefore, applications that involved bonding materials such as copper, aluminum and brass are very common. Materials such as lead, zinc and tin cannot be welded due their high lubricity. The oscillations during ultrasonic welding would only lead to smoothing of the bonding area, not to bonding of the molecules. Even coatings which contain these materials are troublesome and will at the very least will lead to variations in the quality of the weld.

As previously described, the part facing the sonotrode is accelerated and decelerated under pressure using high frequencies. As the available energy for generators with up to approximately 4 kW is limited, the size or the mass of the part to be welded has to be considered. As a general rule parts up to max. 10 grams can be welded. This value can be exceeded if the material is relatively soft so that the material deformation in the welding area is absorbed. The part which faces the anvil, i.e. away from the sonotrode, can be of any weight.

In order to introduce the energy more efficiently and consistently and to better accommodate the parts to be welded, design modifications may be required. For weldings which require a large surface bond, the use of one or several energy directors (welding studs) is recommended.

If the parts to be welded have an unfavorable shape, the oscillation energy can continue to flow inside the workpiece itself and produce a nodal point (wave) which may lead to distortion or even break the part in this area. During the design phase, the shape of the parts has to be studied in this context by conducting welding tests.

If a design change at this stage is not possible, the remedy could be a slight modification of the welding fixture and the anvil. As an alternative, the parts can be specially clamped in certain areas and the energy concentrated in local areas.

When several spots-weldings are carried out on one part, nodal points may form in places where they are undesirable. This requires proper countermeasures to be taken, if necessary.

In order to obtain a constant weld quality it is important to have a uniform surface. Slight contamination is usually acceptable, but heavily oiled, greased or oxidized parts impair the welding process to such an extent that the previously defined amount of energy cannot be introduced into the process, even if the welding time is increased.

Coated surfaces are likewise to be evaluated to determine their suitability for welding. Nickel-plating, silver-coating, copper-coating or aluminum-plating often have a positive effect. Negative effects are known from tin-plating, zinc-coating and from chromium-plated surfaces. On the other hand, materials which are less suitable for ultrasonic metal welding can obtain good welding qualities by applying a suitable surface layer (for example, the application of a nickel or galvanized copper layer on steel).

A major area of application for ultrasonic metal welding is the bonding of enamel copper wires such as those used for coils, transformers, etc. Based on today's technology, good quality, pipeless and homogeneous welds without having to first strip the enamel are rare. The decisive factor here is the quality of the enamel as well as its proportion to the total weld.

Ultrasonic metal welding is already widely used in industry. This means that there is significant application experience in this field. The most important factor is the weldability of the materials and their alloy components as well as the shape of the parts.

When carrying out welding tests with the help of proper lab tools, the quality of the tested welds can be ascertained. In addition, the experience gained is also the basis for the design and construction of welding tools as well as for possible material or design changes. This makes the applications test a vital prerequisite if ultrasonic welding equipment is to produce reliable quality.